The present invention relates to a device and a method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection.
In mobile radio communication systems, higher usage efficiency of the radio wave frequency is desired for increasing the subscriber capacity of the mobile radio communication systems. For attaining the objects, mobile radio communication systems applying pilot synchronous detection to CDMA (Code Division Multiple Access) have been proposed. In the pilot synchronous detection, frequency difference (shift) between a received radio signal and a radio reception device (a local oscillator of the radio reception device) has to be corrected and eliminated. Especially, techniques capable of correcting the frequency shift in a wide frequency range (bandwidth) and with high accuracy are now being required in order to implement mobile radio communication devices capable of receiving radio signals from base stations correctly.
For the frequency correction in the pilot synchronous detection, pilot symbol blocks (each of which is composed of a predetermined number of pilot symbols) are inserted periodically in a transmitted radio signal. Generally, the frequency correction is executed by detecting phase shifts between adjacent pilot symbol blocks and taking the average of the phase shifts. The xe2x80x9cphase shift between adjacent pilot symbol blocksxe2x80x9d means a phase shift (relative to a local oscillation signal outputted by the local oscillator of the radio reception device) which occurs between the adjacent pilot symbol blocks. When there is no frequency difference between the transmitted (received) radio signal and the local oscillator of the radio reception device, each symbol of the received radio signal is received on each cycle of the local oscillation signal correctly, and thus the phase shift becomes 0. On the other hand, when a frequency difference exists between the transmitted (received) radio signal and the local oscillator, a certain amount of phase shift occurs. The averaged phase shift is used by a feedback circuit for correcting the frequency of the local oscillation signal (which is used for the detection of the received radio signal) which is outputted by the local oscillator of the radio reception device. In the frequency correction method using the phase shift between the pilot symbol blocks, the frequency correction can be executed correctly if the phase shift between adjacent pilot symbol blocks is within xc2x1180xc2x0. In the case of the frequency correction method using the phase shift between the pilot symbol blocks, high accuracy frequency correction can be executed even though correctable frequency range is narrow.
Meanwhile, if another frequency correction method using the phase shift between adjacent pilot symbols in a pilot symbol block is employed, the frequency correction can be executed correctly if the phase shift between adjacent pilot symbols is within xc2x1180xc2x0. In the case of the frequency correction method using the phase shift between the pilot symbols, frequency correction with a wide correctable frequency range can be implemented even though the accuracy of the frequency correction is lower.
Further, there has been proposed a frequency correction method for realizing high speed extraction and high trackability by combining the above two frequency correction methods together (T. Watanabe et al. xe2x80x9cA performance of AFC for W-CDMA mobile stationxe2x80x9d, Proceedings of the 1998 Communications Society Conference of IEICE (the Institute of Electronics, Information and Communication Engineers (Japan)), B-5-146 (1998)). FIG. 1 is a schematic block diagram showing a frequency correction device employing the conventional frequency correction method of the document.
In the conventional frequency correction method, pilot symbols are extracted from a despread signal (received signal after despreading (descrambling)) and thereafter two-mode frequency correction is executed. First, the frequency correction is executed in mode #1, in which the phase shifts between adjacent pilot symbols in a pilot symbol block (inter-symbol phase shifts) are detected, the average of the inter-symbol phase shifts is figured out, and frequency correction is executed by use of the averaged inter-symbol phase shift. After a predetermined time has passed, the frequency correction mode is switched into mode #2, in which the phase shifts between adjacent pilot symbol blocks (inter-pilot block phase shifts) are detected, the average of the inter-pilot block phase shifts is figured out, and frequency correction is executed by use of the averaged inter-pilot block phase shift.
However, in the above conventional frequency correction method successively employing the two frequency correction methods, the pilot synchronous detection becomes impossible if the phase shift between adjacent pilot symbol blocks becomes xc2x1180xc2x0 or more after the switching into the mode #2. In the above conventional frequency correction method, the accuracy of the frequency correction could be improved by first executing coarse frequency correction based on the detection of the inter-symbol phase shifts (mode #1) and thereafter executing fine frequency correction based on the detection of the inter-pilot block phase shifts (mode #2). However, the timing for switching the frequency correction mode from mode #1 to mode #2 has to be provided properly and correctly in the conventional frequency correction method, and the frequency correction becomes incorrect if the switching into the inter-pilot block phase shift detection (mode #2) is executed before the frequency correction (inter-pilot block phase shift) converges and decreases within xc2x1180xc2x0. Further, if signal propagation status changed during the data communication based on the inter-pilot block phase shift and the frequency difference (inter-pilot block phase shift) increased xc2x1180xc2x0 or more, frequency correction thereafter can not be executed correctly.
It is therefore the primary object of the present invention to provide a frequency correction device and a frequency correction method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection, by which the frequency correction can be executed with high accuracy even if a large frequency difference (phase shift) occurred.
Another object of the present invention is to provide a frequency correction device and a frequency correction method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection, by which a large frequency difference can be corrected accurately regardless of whether it is at the beginning of communication or in the middle of communication, without the need of providing proper and correct timing for mode switching etc.
In accordance with a first aspect of the present invention, there is provided a frequency correction device for correcting the frequency of a local oscillator of a radio reception device according to pilot synchronous detection, comprising a pilot symbol detection means, an inter-symbol phase shift detection means, an inter-pilot block phase shift detection means, a frequency correction determination means and a frequency correction means. The pilot symbol detection means detects and extracts pilot symbols from a received radio signal after despreading. The inter-symbol phase shift detection means detects an inter-symbol phase shift between adjacent pilot symbols in a pilot symbol block. The inter-pilot block phase shift detection means detects an inter-pilot block phase shift between adjacent pilot symbol blocks. The frequency correction determination means determines a frequency correction based on the inter-symbol phase shift which has been detected by the inter-symbol phase shift detection means and the inter-pilot block phase shift which has been detected by the inter-pilot block phase shift detection means. The frequency correction means executes frequency correction to the local oscillator based on the frequency correction which has been determined by the frequency correction determination means.
In accordance with a second aspect of the present invention, in the first aspect, the frequency correction determination means obtains a frequency correction based on inter-symbol phase shift as:
frequency correction (symbol)=(inter-symbol phase shift)/2xcfx80xc3x97SR (Hz)
where xe2x80x9cSRxe2x80x9d is the symbol rate, and obtains frequency correction candidates based on inter-pilot block phase shift as:
frequency correction (block, candidate)={(inter-pilot block phase shift)xc2x12mxcfx80}/2xcfx80xc3x97PBR (Hz),
where xe2x80x9cPBRxe2x80x9d is the pilot symbol block rate and xe2x80x9cmxe2x80x9d is a non-negative integer, and selects one of the frequency correction candidates (frequency correction (block, candidate)) that is the nearest to the frequency correction based on inter-symbol phase shift (frequency correction (symbol)) as the frequency correction.
In accordance with a third aspect of the present invention, in the first aspect, the frequency correction device further comprises an inter-symbol phase shift averaging means and an inter-pilot block phase shift averaging means. The inter-symbol phase shift averaging means takes the average of the inter-symbol phase shifts which have been detected by the inter-symbol phase shift detection means. The inter-pilot block phase shift averaging means takes the average of the inter-pilot block phase shifts which have been detected by the inter-pilot block phase shift detection means. The frequency correction determination means determines the frequency correction based on the averaged inter-symbol phase shift (xcex94(symbol ave)) which has been obtained by the inter-symbol phase shift averaging means and the averaged inter-pilot block phase shift (xcex94(block ave)) which has been obtained by the inter-pilot block phase shift averaging means.
In accordance with a fourth aspect of the present invention, in the third aspect, the frequency correction determination means obtains a frequency correction based on inter-symbol phase shift as:
frequency correction (symbol)=xcex94(symbol ave)/2xcfx80xc3x97SR (Hz)
(where xe2x80x9cSRxe2x80x9d is the symbol rate), and obtains frequency correction candidates based on inter-pilot block phase shift as:
frequency correction (block, candidate)={xcex94(block ave)xc2x12 mxcfx80}/2xcfx80xc3x97PBR (Hz)
(where xe2x80x9cPBRxe2x80x9d is the pilot symbol block rate and xe2x80x9cmxe2x80x9d is a non-negative integer), and selects one of the frequency correction candidates (frequency correction (block, candidate)) that is the nearest to the frequency correction based on inter-symbol phase shift (frequency correction (symbol)) as the frequency correction.
In accordance with a fifth aspect of the present invention, in the third aspect, the inter-symbol phase shift averaging means includes a first multiplier, a second multiplier, an adder and an accumulator. The first multiplier multiplies the inter-symbol phase shift detected by the inter-symbol phase shift detection means by (1xe2x88x92xcex1) (xcex1: oblivion coefficient). The second multiplier multiplies the output of the accumulator by xcex1. The adder adds the xcex1-multiplied output of the accumulator and the (1xe2x88x92xcex1)-multiplied inter-symbol phase shift together and outputs the addition as the averaged inter-symbol phase shift (xcex94(symbol ave)). The accumulator accumulates the addition outputted by the adder.
In accordance with a sixth aspect of the present invention, in the third aspect, the inter-pilot block phase shift averaging means includes a first multiplier, a second multiplier, an adder and an accumulator. The first multiplier multiplies the inter-pilot block phase shift detected by the inter-pilot block phase shift detection means by (1xe2x88x92xcex2) (xcex2: oblivion coefficient). The second multiplier multiplies the output of the accumulator by xcex2. The adder adds the xcex2-multiplied output of the accumulator and the (1xe2x88x92xcex2)-multiplied inter-pilot block phase shift together and outputs the addition as the averaged inter-pilot block phase shift (xcex94(block ave)). The accumulator accumulates the addition outputted by the adder.
In accordance with a seventh aspect of the present invention, in the third aspect, the inter-symbol phase shift averaging means includes a first multiplier, a second multiplier, a first adder and a first accumulator, and the inter-pilot block phase shift averaging means includes a third multiplier, a fourth multiplier, a second adder and a second accumulator. In the inter-symbol phase shift averaging means, the first multiplier multiplies the inter-symbol phase shift detected by the inter-symbol phase shift detection means by (1xe2x88x92xcex1) (xcex1: oblivion coefficient). The second multiplier multiplies the output of the first accumulator by xcex1. The first adder adds the a xcex1-multiplied output of the first accumulator and the (1xe2x88x92xcex1)-multiplied inter-symbol phase shift together and outputs the addition as the averaged inter-symbol phase shift (xcex94(symbol ave)). The first accumulator accumulates the addition outputted by the first adder. Meanwhile, in the inter-pilot block phase shift averaging means, the third multiplier multiplies the inter-pilot block phase shift detected by the inter-pilot block phase shift detection means by (1xe2x88x92xcex2) (xcex2: oblivion coefficient). The fourth multiplier multiplies the output of the second accumulator by xcex2. The second adder adds the xcex2-multiplied output of the second accumulator and the (1xe2x88x92xcex2)-multiplied inter-pilot block phase shift together and outputs the addition as the averaged inter-pilot block phase shift (xcex94(block ave)). The second accumulator accumulates the addition outputted by the second adder.
In accordance with an eighth aspect of the present invention, in the first aspect, the frequency correction means includes a D/A conversion means for executing D/A conversion to the output of the frequency correction determination means.
In accordance with a ninth aspect of the present invention, there is provided a frequency correction method for correcting the frequency of a local oscillator of a radio reception device according to pilot synchronous detection. The frequency correction method comprises a pilot symbol detection step, an inter-symbol phase shift detection step, an inter-pilot block phase shift detection step, a frequency correction determination step and a frequency correction step. In the pilot symbol detection step, pilot symbols are detected and extracted from a received radio signal after despreading. In the inter-symbol phase shift detection step, an inter-symbol phase shift between adjacent pilot symbols in a pilot symbol block is detected. In the inter-pilot block phase shift detection step, an inter-pilot block phase shift between adjacent pilot symbol blocks is detected. In the frequency correction determination step, a frequency correction is determined based on the inter-symbol phase shift which has been detected in the inter-symbol phase shift detection step and the inter-pilot block phase shift which has been detected in the inter-pilot block phase shift detection step. In the frequency correction step, frequency correction is executed to the local oscillator based on the frequency correction which has been determined in the frequency correction determination step.
In accordance with a tenth aspect of the present invention, in the frequency correction determination step of the ninth aspect, a frequency correction based on inter-symbol phase shift is obtained as:
frequency correction (symbol)=(inter-symbol phase shift)/2xcfx80xc3x97SR (Hz)
(where xe2x80x9cSRxe2x80x9d is the symbol rate), and frequency correction candidates based on inter-pilot block phase shift are obtained as:
frequency correction (block, candidate)={(inter-pilot block phase shift)xc2x12mxcfx80}/2xcfx80xc3x97PBR (Hz)
(where xe2x80x9cPBRxe2x80x9d is the pilot symbol block rate and xe2x80x9cmxe2x80x9d is a non-negative integer), and one of the frequency correction candidates (frequency correction (block, candidate)) that is the nearest to the frequency correction based on inter-symbol phase shift (frequency correction (symbol)) is selected as the frequency correction.
In accordance with an eleventh aspect of the present invention, in the ninth aspect, the frequency correction method further comprises an inter-symbol phase shift averaging step and an inter-pilot block phase shift averaging step. In the inter-symbol phase shift averaging step, the average of the inter-symbol phase shifts which have been detected in the inter-symbol phase shift detection step is taken. In the inter-pilot block phase shift averaging step, the average of the inter-pilot block phase shifts which have been detected in the inter-pilot block phase shift detection step is taken. In the frequency correction determination step, the frequency correction is determined based on the averaged inter-symbol phase shift (xcex94(symbol ave)) which has been obtained in the inter-symbol phase shift averaging step and the averaged inter-pilot block phase shift (xcex94(block ave)) which has been obtained in the inter-pilot block phase shift averaging step.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, in the frequency correction determination step, a frequency correction based on inter-symbol phase shift is obtained as:
frequency correction (symbol)=xcex94(symbol ave)/2xcfx80xc3x97SR (Hz)
(where xe2x80x9cSRxe2x80x9d is the symbol rate), and frequency correction candidates based on inter-pilot block phase shift are obtained as:
frequency correction (block, candidate)={xcex94(block ave)xc2x12mxcfx80}/2xcfx80xc3x97PBR (Hz)
(where xe2x80x9cPBRxe2x80x9d is the pilot symbol block rate and xe2x80x9cmxe2x80x9d is a non-negative integer), and one of the frequency correction candidates (frequency correction (block, candidate)) that is the nearest to the frequency correction based on inter-symbol phase shift (frequency correction (symbol)) is selected as the frequency correction.
In accordance with a thirteenth aspect of the present invention, in the eleventh aspect, the inter-symbol phase shift averaging step includes a first multiplication step, a second multiplication step, an addition step and an accumulation step. In the first multiplication step, the inter-symbol phase shift detected in the inter-symbol phase shift detection step is multiplied by (1xe2x88x92xcex1) (xcex1: oblivion coefficient). In the second multiplication step, the result of the accumulation step is multiplied by xcex1. In the addition step, the xcex1-multiplied result of the accumulation step and the (1xe2x88x92xcex1)-multiplied inter-symbol phase shift are added together and the addition is regarded as the averaged inter-symbol phase shift (xcex94(symbol ave)). In the accumulation step, the addition obtained in the addition step is accumulated.
In accordance with a fourteenth aspect of the present invention, in the eleventh aspect, the inter-pilot block phase shift averaging step includes a first multiplication step, a second multiplication step, an addition step and an accumulation step. In the first multiplication step, the inter-pilot block phase shift detected in the inter-pilot block phase shift detection step is multiplied by (1xe2x88x92xcex2) (xcex2: oblivion coefficient). In the second multiplication step, the result of the accumulation step is multiplied by xcex2. In the addition step, the xcex2-multiplied result of the accumulation step and the (1xe2x88x92xcex2)-multiplied inter-pilot block phase shift are added together and the addition is regarded as the averaged inter-pilot block phase shift (xcex94(block ave)). In the accumulation step, the addition obtained in the addition step is accumulated.
In accordance with a fifteenth aspect of the present invention, in the eleventh aspect, the inter-symbol phase shift averaging step includes a first multiplication step, a second multiplication step, a first addition step and a first accumulation step, and the inter-pilot block phase shift averaging step includes a third multiplication step, a fourth multiplication step, a second addition step and a second accumulation step. In the first multiplication step, the inter-symbol phase shift detected in the inter-symbol phase shift detection step is multiplied by (1xe2x88x92xcex1) (xcex1: oblivion coefficient). In the second multiplication step, the result of the first accumulation step is multiplied by xcex1. In the first addition step, the xcex1-multiplied result of the first accumulation step and the (1xe2x88x92xcex1)-multiplied -multiplied inter-symbol phase shift are added together and the addition is regarded as the averaged inter-symbol phase shift (xcex94(symbol ave)). In the first accumulation step, the addition obtained in the first addition step is accumulated. In the third multiplication step, the inter-pilot block phase shift detected in the inter-pilot block phase shift detection step is multiplied by (1xe2x88x92xcex2) (xcex2: oblivion coefficient). In the fourth multiplication step, the result of the second accumulation step is multiplied by xcex2. In the second addition step, the xcex2-multiplied result of the second accumulation step and the (1xe2x88x92xcex2)-multiplied inter-pilot block phase shift are added together and the addition is regarded as the averaged inter-pilot block phase shift (xcex94(block ave)). In the second accumulation step, the addition obtained in the second addition step is accumulated.
In accordance with a sixteenth aspect of the present invention, in the ninth aspect, the frequency correction step includes a D/A conversion step in which D/A conversion is executed to the result of the frequency correction determination step.
In accordance with seventeenth through twenty-fourth aspects of the present invention, there are provided computer-readable record mediums storing programs for instructing a computer, an MPU (MicroProcessor Unit), a DSP (Digital Signal Processor), etc. to execute the frequency correction methods of the ninth through sixteenth aspects of the present invention.